1. Field of the Invention
The present invention generally relates to electrochemical cell design and fabrication and, more particularly, to techniques for filling the electrochemical cells with electrolyte solutions.
2. Description of the Related Art
In the manufacture of electrochemical power supplies, there is a great deal of interest in developing better and more efficient methods for storing energy in electrochemical cells having high energy density and high power density. Increasing power per unit volume and increasing discharge characteristics depends on the ability to fabricate thinner electrodes and thinner separators. Accordingly, there has been an increasing effort to develop such thin and improved performance electrochemical cells using cost-effective methods.
An electrochemical cell uses its cathode and anode electrodes to generate an electric current. The electrodes are separated from one another by a porous separator element. During the manufacture of the electrochemical cell, the anode, the separator and the cathode are laminated together to form a laminated cell structure. This laminated electrochemical cell structure is then filled with an electrolyte solution to maintain the flow of ionic conduction between the electrodes. The amount and the distribution of electrolyte within the cell volume is important for the cell's overall performance. In fact, flooding or depletion of the cell electrolyte severely impairs cell performance and may cause failures.
In the prior art, there are several methods for filling electrochemical cells with electrolyte. In one exemplary prior art process, the laminated cell structure is rolled on a mandril to yield a cylindrical spiral wound roll, which is referred to as "jelly roll". The roll is then placed into a container having an electrolyte fill port. Once the container is sealed, electrolyte is injected into the container through the fill port and subsequently the fill port is sealed. In another prior art process, the laminated cell structure is maintained in a flat prismatic configuration and soaked in an electrolyte solution until the porous laminated structure is flooded. Subsequently, the cell is placed into a cell container and the container is sealed.
However, as state-of-the-art electrodes and separators get thinner to increase the power density of the cell, such established prior art processes for filling cells become less efficient. The state of the art thin electrochemical cells include micro-porous cell components (i.e., separators and electrodes). These components contain smaller pores which inhibit the transport of liquid electrolyte throughout the cell. For example, the transport of the electrolyte in the porous cell structure may be significantly reduced or inhibited if the surface tension of the electrolyte is not significantly lower than the surface energy of the porous media. Additionally, as liquid electrolyte enters the pore structure of the electrodes and separator, gas (typically air) in the pore structure must be displaced with the electrolyte. However, thin separators also restrict the egress of gas from the cell. These conditions greatly increase the amount of time required to fill the cell with electrolyte, and the difficulty in assuring a uniform filling of the separator material.
In order to overcome these problems, several modifications in the prior art processes have been suggested. In one technique, the thin laminated structure is placed into a container having an adequate headspace. The head space is to accommodate the overflow of electrolyte above the cell until the electrolyte is drawn into the separator and porous electrode structures. However, due to the slow displacement of gas in the porous structure, this technique is time consuming and increases the manufacturing cost.
Alternatively, to decrease the amount of time required for this filling process, surfactants can be added to the electrolyte to reduce the surface tension of the electrolyte and improve the wetting of the porous cell components in the cell. In addition, cosolvents may also be added to the electrolyte to reduce the viscosity of the liquid and thereby increase the flow of electrolyte into the porous cell components of the cell. However, preparing electrolyte solutions with such chemistry adds materials to the electrolyte that do not contribute to the electrochemical performance of the cell, but do add to the manufacturing cost. In an alternative approach, a cell may be filled under vacuum to eliminate the slow displacement of gas from the pore structure when the electrolyte is added.
Additional complication, each current collector in the stack of cell must be filled. In this type of construction, each current collector in the stack (except the collectors at the ends of the stack) must serve as a positive electrode on one side and as a negative electrode on the other side. In bipolar stacks, the electrolyte on the positive side of the collectors must be isolated from the electrolyte on the negative side of the collectors to prevent ionic shunt currents between adjacent cells. Furthermore, the electrolyte in each cell must be isolated from all other cells in the stack to prevent electrolysis of the electrolyte. It is also very important that each cell in the bipolar stack receive the same amount of electrolyte, as cells with disproportionately less electrolyte exhibit slightly higher resistance, and cycle between slightly larger voltage limits. These cells fail prematurely with respect to other cells in the stack and cause a failure of the entire stack. Electrolyte deficient cells may also be driven into reversal by the other cells in the stack leading again to premature failure of the entire stack. These conditions warrant that each cell in the stack must have a perimeter seal at the edge of the current collectors to retain the electrolyte in the cell.
For the above reasons, the process of adding electrolyte to the bipolar stacks of cells require special methods. In one prior art method, a fill port is included in each perimeter seal of the stack of cells. A manifold of tubing connects each fill port to an electrolyte supply-reservoir. During the filling process, the electrolyte from the reservoir is injected under pressure via fill ports into each cell. In this approach, cells may be evacuated prior to injection of the electrolyte to remove trapped gas and to aid in the distribution of the electrolyte throughout the cell. Once cells are filled with electrolyte, the manifold can be left in place. However, valves to each cell must be can be removed and, each fill port can be sealed individually.
In another prior art method, a fill port is included in each perimeter seal of each cell. The aligned fill ports in the bipolar stack ate submerged in electrolyte and a vacuum is applied to extract the gas from the cell components. As the stack is brought back to atmospheric pressure, the electrolyte is drawn into the individual cells. The excess electrolyte is then cleaned from the outside of the cell stack, and each fill port is sealed.
In yet another prior art method, each side of the bipolar current collector is fitted with an elastic gasket to form a shallow cup which contains the anode and cathode materials on either side of the current collector. In this method, the anode or the cathode or both electrodes may be prepared as slurries comprising an active electrode material and an electrolyte. Carbon powder may also be added to this slurry to enhance the electronic conductivity of the electrode. These electrolyte soaked bipolar electrodes are stacked between porous separators under pressure. The elastic gaskets hold the separators in place and seal the perimeters of the cells. Alternatively, the gaskets are made of thermal plastics which are thermally sealed after the bipolar electrodes are stacked in series with the separators. In these cases fill ports are not required.
For all of these methods, attention must be directed to the distribution of the electrolyte in the cell during closure of the cell and closure of the fill ports. Typically, cells and fill ports are closed or sealed with adhesives, thermal plastics, elastomers (crimp seals), or by welding when metal containers (cans) are used. In order to provide a leakage free seal, all joining surfaces must be free of electrolyte.
There is a need in the electrochemical cell manufacturing industry for processes for filling thin electrochemical cells with electrolyte without using fill ports or evacuation apparatus. Such processes must quickly and accurately meter and uniformly distribute the electrolyte throughout the pore structure of the electrodes and separator without contacting the surfaces of the perimeter seals or other joining surfaces.